1. Field of the Invention
The present invention generally relates to the photo detectors and, more particularly, to the gallium-nitride based ultraviolet photo detectors.
2. The Prior Arts
Currently there are three commonly known approaches to building photo detectors that can convert light signals to electrical signals: photo multipliers (PMTs) based on vacuum tubes; photo detectors made of silicon (Si); and photo detectors made of gallium-nitride (GaN) based compound semiconductors.
Among the three approaches, PMTs have a number of disadvantages such as high production cost, high operation voltage, and the fragility of vacuum tubes. The Si-based photo detectors, on the other hand, are easer to manufacture, less costly in production, and operated with lower voltage. However, despite their rather superior performance in detecting lights with longer wavelengths such as visible lights and infrared lights, Si-based photo detectors are less sensitive to lights with shorter wavelengths such as the ultraviolet (UV) lights. In contrast, photo detectors made of GaN-based compound semiconductors, as the GaN-based materials can possess wider band gaps, are very good at detecting UV lights. In addition, by controlling the GaN-based materials' compositions to alter their band gaps, the GaN-based photo detectors can be made to detect UV lights within a specific range of wavelengths.
FIG. 1 is a response characteristics graph of a GaN-based UV photo detector. As can be seen from FIG. 1, UV photo detectors are especially responsive to UV lights whose wavelengths are between 300 nm and 370 nm; this kind of sensitivity is not available from Si-based photo detectors. Therefore, for applications that require this kind of fast response, GaN-based UV photo detectors are the most appropriate candidates.
One of the major problems to known GaN-based semiconductor devices is that there is often an incompatibility in terms of lattice constants within the GaN-based semiconductor devices' layering structure. Such discrepancies in the lattice structures develop and accumulate excessive stress, causing an inferior epitaxial structure of the GaN-based semiconductor devices and thereby an unsatisfactory device performance. In addition, GaN-based photo detectors, on one hand, should have band gaps wide enough to be responsive to UV lights within a specific range of wavelengths and, on the other hand, GaN-based photo detectors should have narrower band gaps so as to form better ohmic contacts with the photo detectors' electrodes. Moreover, problems such as how to reduce the reflection of UV lights from the surface of the GaN-based UV photo detectors and how to increase the GaN-based UV photo detectors' photoelectric conversion efficiency also need to be addressed, so that GaN-based UV photo detectors can have practical values.